The term "fermentation" as used herein means any of a group of chemical reactions induced by living or nonliving biocatalysts. The term "culture" as used herein means the suspension or attachment of any such biocatalyst in or covered by a liquid medium for the purpose of maintaining chemical reactions. The term "biocatalysts" as used herein, includes enzymes, vitamins, enzyme aggregates, immobilized enzymes, subcellular components, prokaryotic cells, and eukaryotic cells. The term "centrifugal force" means a centripetal force resulting from angular rotation of an object when viewed from a congruently rotating frame of reference.
The culture of microbial cells (fermentation) or animal and plant cells (tissue culture) are central to a multiplicity of commercially-important chemical and biochemical production processes. Living cells are employed in these processes as a result of the fact that living cells, using generally easily obtainable starting materials, can economically synthesize commercially-valuable chemicals.
Fermentation involves the growth or maintenance of living cells in a nutrient liquid media. In a typical batch fermentation process, the desired micro-organism or eukaryotic cell is placed in a defined medium composed of water, nutrient chemicals and dissolved gases, and allowed to grow (or multiply) to a desired culture density. The liquid medium must contain all the chemicals which the cells require for their life processes and also should provide the optimal environmental conditions for their continued growth and/or replication. Currently, a representative microbial cell culture process might utilize either a continuous stirred-tank reactor or a gas-fluidized bed reactor in which the microbe population is suspended in circulating nutrient media. Similarly, in vitro mammalian cell culture might employ a suspended culture of cells in roller flasks or, for cells requiring surface attachment, cultures grown to confluence in tissue culture flasks containing nutrient medium above the attached cells. The living cells, so maintained, then metabolically produce the desired product(s) from precursor chemicals introduced into the nutrient mixture. The desired product(s) are either purified from the liquid medium or are extracted from the cells themselves.
Examples of methods employing fermentations of cells growing in either agitated aqueous suspension or with surface attachment are described, for example, in U.S. Pat. Nos. 3,450,598; 3,843,454; 4,059,485; 4,166,768; 4,178,209; 4,184,916; 4,413,058; and 4,463,019. Further reference to these and other such conventional cell culturing techniques may be found in such standard texts as Kruse and Patterson, Tissue Culture Methods and Applications, Academic Press, New York, 1977; and Collins and Lyne's Microbiological Methods, Butterworths, Boston, 1989.
There are a number of disadvantages inherent in such typical fermentation processes. On a commercial scale, such processes require expensive energy expenditures to maintain the large volumes of aqueous solution at the proper temperature for optimal cell viability. In addition, because the metabolic activity of the growing cell population causes decreases in the optimal levels of nutrients in the culture media as well as causing changes in the media pH, the process must be continuously monitored and additions made to maintain nutrient concentration and pH at optimal levels.
In addition, the optimal conditions under which the desired cell type may be cultured is usually near the optimal conditions for the growth of many other undesirable cells or microorganisms. Extreme care and expense must be taken to initially sterilize and to subsequently exclude undesired cell types from gaining access to the culture medium. Next, such fermentation methods, particularly those employing aerobic organisms, are quite often limited to low yields of product or low rates of product formation as a result of the inability to deliver adequate quantities of dissolved oxygen to the metabolizing organism. Finally, such batch or semi-batch processes can only be operated for a finite time period before the buildup of excreted wastes in the fermentation media require process shutdown followed by system cleanup, resterilization, and a re-start.
The high costs associated with the preparation, sterilization, and temperature control of the large volumes of aqueous nutrient media needed for such cultures has led to the development of a number of processes whereby the desired cell type or enzyme can be immobilized in a much smaller volume through which smaller quantities of nutrient media can be passed. Cell immobilization also allows for a much greater effective density of cell growth and results in a much reduced loss of productive cells to output product streams. Thus, methods and processes for the immobilization of living cells are of considerable interest in the development of commercially valuable biotechnologies.
An early method for the immobilization of cells or enzymes involved the entrapment of such biocatalysts on or within dextran, polyacrylamide, nylon, polystyrene, calcium alginate, or agar gel structures. Similarly, the ability of many animal cells to tenaciously adhere to the external surface of spherical polymeric "microcarrier beads" has likewise been exploited for the immobilization of such cells. These gel- or bead-immobilization methods effectively increase the density of the biocatalyst-containing fraction, thereby effectively trapping these structures in the lower levels of relatively slow flowing bioreactor chambers. Such gel-entrapment or microcarrier-immobilized methods are taught, for example, in U.S. Pat. Nos. 3,717,551; 4,036,693; 4,148,689; 4,189,534; 4,203,801; 4,237,033; 4,237,218; 4,266,032; 4,289,854; 4,293,654; 4,335,215; and 4,898,718. More background information on cell immobilization techniques is discussed in Chibata, et al., "Immobilized Cells in the Preparation of Fine Chemicals", Advances in Biotechnological Processes, Vol. I, A.R. Liss, Inc., New York, 1983. See also Clark and Hirtenstein, Ann. N.Y. Acad. Sci. 369, 33-45 (1981), for more background information on microcarrier culture techniques.
These immobilization methods suffer from a number of drawbacks. First, such entrapment of cells within gels has been shown to interfere with the diffusion of gases (particularly oxygen and carbon dioxide) into and out of the cell environment, resulting in either low cell growth (reduced oxygen input) or gel breakage (high internal CO.sub.2 pressure). In addition, the poor mechanical properties and high compressibility of gel-entrapment media lead to unacceptably high pressure problems in packed bed bioreactors. Similarly, the crushing of microcarrier beads and the destruction of attached cells by hydraulic shear forces in agitated tank reactors (necessary to increase gas exchange) leads to reduced viability and productivity.
Another method for the immobilization of living cells or enzymes currently in use involves the use of packed-bed reactors. In these methods, free cells or cells bound to microcarrier beads are suspended in a rigid or semi-rigid matrix which is placed within a culture bioreactor. The matrix possesses interstitial passages for the transport of liquid nutrient media into the reactor, similarly disposed passages for the outflow of liquid media and product chemicals, and similar interstitial passages through which input and output gases may low. Bioreactors of this type include the vat type, the packed-column type, and the porous ceramic-matrix type bioreactor. Such methods are taught, for example, in U.S. Pat. Nos. 4,203,801; 4,220,725; 4,279,753; 4,391,912; 4,442,206; 4,537,860; 4,603,109; 4,693,983; 4,833,083; 4,898,718; and 4,931,401.
These methods of immobilization all suffer from a number of problems, particularly when scaled up to production size. First of all, such bioreactors are subject to concentration gradients. That is, the biocatalysts nearer the input nutrient liquid feed see higher substrate levels than those farther downstream. Conversely, those biocatalysts farther from the input liquid stream (and closer to the exit liquid port) see increased concentrations of waste products and often suffer suboptimal environmental conditions, such as a changed pH and/or lowered dissolved oxygen tension. Next, such bioreactors are particularly susceptible to the "bleeding" of biocatalysts detached from the matrix (or released by cell division), with the result that output ports become clogged with cells and/or debris. The result is an unacceptable pressure drop across the bioreactor which causes further deterioration of production. Finally, such vertical packed-bed bioreactors in which glass or other microcarrier beads are packed subject the lower portion of the bed to the weight of those beads above, with the inevitable result that both beads and cells are crushed by the sheer weight and number of beads need for production-scale columns.
A more recently-developed class of methods for cell immobilization involves the confinement of the desired cells between two synthetic membranes. Typically, one membrane is microporous and hydrophilic and in contact with the aqueous nutrient media, while the opposing membrane is ultraporous and hydrophobic and in contact with a flow of air or an oxygen-enriched gas. Such processes thus provide the cells with an environment in which nutrient liquid input and waste liquid output can occur through channels separate from the cell-containing space and similarly provide gaseous input and output through similarly disposed channels, again separate from the cell-containing space. Embodiments of methods of this class have utilized stacks of many flat membranes forming a multiplicity of cell compartments, have utilized series of synthetic membrane bags, one within the other, and have utilized spirally-wound membrane configurations. Such methods are taught, for example, in U.S. Pat. Nos. 3,580,840; 3,843,454; 3,941,662; 3,948,732; 4,225,671; 4,661,455; 4,748,124; 4,764,471; 4,839,292; 4,895,806; and 4,937,196.
Unfortunately, there are a number of problems with such methods, particularly for any commercial, large-scale usage. First, such devices in which a multiplicity of membranes are stacked in series are quite costly to manufacture and are extremely difficult to correctly assemble. Next, the requirement that the membrane which separates the nutrient channels from the immobilized cells be hydrophilic necessarily results in cell attachment across pores and/or pore clogging by insolubles in the nutrient feed or waste output liquids which wet this membrane. The result is the development over time of "dead pockets" where cell growth cannot occur. This situation greatly reduces the effective cell concentration and lowers product yield. Finally, these methods involve devices with a large number of inlet and outlet ports and external fittings which substantially increase both cost and the probability that leakage and contamination will occur.
Another class of methods for cell immobilization involves the employment of capillary hollow fibers (usually configured in elongated bundles of many fibers) having micropores in the fiber walls. Typically, cells are cultured in a closed chamber into which the fiber bundles are placed. Nutrient aqueous solutions flow freely through the capillary lumena and the hydrostatic pressure of this flow results in an outward radial perfusion of the nutrient liquid into the extracapillary space in a gradient beginning at the entry port. Similarly, this pressure differential drives an outward flow of "spent" media from the cell chamber back into the capillary lumena by which wastes are removed. Cells grow in the extracapillary space either in free solution or by attachment to the extracapillary walls of the fibers. Typically, oxygen is dissolved into the liquid fraction of the extracapillary space by means of an external reservoir connected to this space via a pump mechanism. Waste products in the intracapillary space may be removed by reverse osmosis in fluid circulated outside of the cell chamber. Such methods are taught, for example, by U.S. Pat. Nos. 3,821,087; 3,883,393; 3,997,396; 4,087,327; 4,184,922; 4,201,845; 4,220,725; 4,442,206; 4,722,902; 4,804,628; and 4,894,342. There are a number of difficulties with the use of methods based on capillary hollow fiber cell immobilization methods.
Cracauer et al. (U.S. Pat. No. 4,804,628) have extensively documented these difficulties. Among them are: (1) An excessive pressure drop through the fiber assembly. The fragile nature of the fibers results in complete breakdown if fiber of production-scale length is required; (2) Adverse chemical gradients occur within the cell compartment. Gradients of nutrients and waste products often occur in such chambers; (3) Formation of anoxic pockets and discrete disadvantageous microenvironments occur within the cell chamber. Because of the inaccessibility of liquids, gases, and cells to all portions of the fiber bundle as a result of their design, not all areas of the cell chamber are equally effective in cell production; and, (4) Mass transfer limitations in nutrient feed and product output increase with time. As cells grow to higher densities, they tend to self-limit the capacities of the hollow fiber chambers (see Col. 1, lines 53-66, of U.S. Pat. No. 4,804,628).
Another class of methods for the mass culture of living cells involves the use of fluidized bed bioreactors. The excellent mixing characteristics and fluid dynamics of this type of mass culture have found usage in both microbial and bead-immobilized animal cell culture. The major disadvantage of fluidized bed methods, and particularly a variant called airlift fermentors, results from the necessity to bubble air or oxygen through the reactor and the resultant presence of a gas-liquid interface throughout the reactor volume. Firstly, the presence of gas bubbles in the flowing liquid disrupts the fluid dynamics which provided the initial advantages of fluidized beds (uniform particle suspension). Next, protein foaming, cell destruction, and the denaturation of nutrients and products occurs at the large gas-liquid interface. Finally, cell washout is almost inevitable in continuous operation, particularly with animal cell culture.
Another class of methods for mass cell culture is known as dual axis, continuous flow bioreactor processing. Such methods are taught by, for example, U.S. Pat. Nos. 5,151,368, 4,296,882, and 4,874,358. In this class of bioreactor, rotation of the reactor chamber about an axis perpendicular to the vertical axis is utilized in order to effect internal mixing of the bioreactor contents while rotation about the vertical axis confines grossly particulate matter at radial distances far from the vertical axis of rotation. Input nutrient liquids and gases are supplied by concentric flexible conduits into the bioreactor and output liquids and gases are removed by similar flexible conduits concentric with the input tubings. While the intended purpose of bioreactors of this class is to allow continuous flow of liquid into and out of a bioreactor chamber in which a combination of solids and liquids is suspended and mixed, such processes are limited to rotational speeds at which effective mixing can occur without appreciable negation by centrifugal forces. As a result, methods of this class are ineffective in the immobilization of low mass micro-organisms, particularly those requiring gaseous nutrients and producing waste gas products. Other similar centrifugal liquid processing apparati are disclosed in U.S. Pat. Nos. 4,113,173, 4,114,802, 4,372,484, and 4,425,112. In each of these latter references, liquid flow through a centrifugal chamber is supplied by flexible tubing extending through the rotational axis.
Another type of bioreactor called a "Nonhomogeneous Centrifugal Film Bioreactor" intended for aerobic cell culture is taught by U.S. Pat. No. 5,248,613. The object of the method is to maximize the "entrainment of the maximum amount of the gaseous phase into the liquid phase" by causing the formation of a thin liquid film to contact the gas phase and further, to centrifugally generate small liquid droplets which fall through a relatively stationary gas phase back into the recirculated bulk liquid phase. There are a number of problems associated with a bioreactor design of this type. First of all, it is a "batch" process. That is, the nutrient liquid phase gradually is depleted of its components while liquid metabolic wastes build up, necessitating a limited culture time. Secondly, the scale of such a bioreactor is limited by the quantity of nutrient gas (such as oxygen) which can be dissolved in the various gas-liquid transfer regions. In the limit, the maximum gas transfer obtainable at atmospheric pressure will determine the maximum cell "load" which can be carried by the reactor system. Next, the lack of any provision for the removal of waste gases (such as carbon dioxide) will result in disruption of both bulk liquid pH as well as cellular productivity as culture periods extend to longer times. Finally, it is extremely doubtful whether or not animal cells subjected to passage through a high flow rate, thin-film liquid region where cell-disrupting surface tension forces are maximal and where limited nutrient availability in the presence of maximum aerobicity will occur would not lead to accelerated productive cell loss.
A final method for the mass immobilization of living cells called "Continuous Centrifugal Bioprocessing" has been taught by Van Wie, et al. (U.S. Pat. No. 4,939,087). In this method cells are "captured" by a velocity gradient in a centrifugal field in order to maintain a culture in a revolving bioreactor chamber into which and out of which liquid flows are pumped. The basic idea upon which the invention of Van Wie et al. is based was first postulated by Lindahl in 1948 (Lindahl, P.E. (1948) Nature (London) 161,648-649) and a U.S. Patent awarded in the same year to MacLeod (U.S. Pat. No. 2,616,619). More recently, Beckman Instruments has developed analytical devices called "Centrifugal Elutriation Systems" based on the general principles of what is termed "Counterflow Centrifugation." The particle and fluid dynamic theory upon which these devices were constructed and refined has been most completely discussed by Sanderson and Bird (Sanderson, R.J. and Bird, K.E. (1977) Methods in Cell Biology, 15, 1-14). As is shown in FIG. 1, the basic idea is to suspend a particle in a spinning bioreactor chamber, which as a consequence of its rotation, imparts a "centrifugal" force to the particle which would normally cause the particle to migrate to longer centrifugal radii. Liquid flow is introduced into the periphery of the spinning chamber (and withdrawn at shorter radii) in order to impart an opposing force which counteracts that of the centrifugal field, with the result that the particle is immobilized at a particular radial distance in a liquid flow. The essence of Sanderson and Bird's mathematical analysis of the particle and fluid dynamics of this process are displayed in FIG. 2. As do all theoretical discussions of centrifugation theory, Sanderson and Bird's analysis begins with the application of Stoke's Law at low Reynolds numbers, an expression which governs the motion of a particle moving through an incompressible fluid (Eqn. 1). Briefly, the law states that the sedimentation velocity (SV) of a non-deformable particle moving through a stationary liquid under the influence of a centrifugal field is proportional to the square of the effective diameter of the particle (d) multiplied by the difference between the density of the particle and the density of the liquid (.rho..sub.p -.rho..sub.m) divided by the product of the liquid viscosity (.eta.) and the "shape constant" of the particle (k, its deviation from sphericity) all times the square of the angular velocity (.omega..sup.2 r) of the rotating system at radius r. As was recognized first by Lindahl, the same equation applies to a stationary particle in a moving liquid flow. The analysis of Sanderson and Bird led to the derivation of Eqn. 2, an expression which states that "there is a radius r.sub.x (defined by evaluation of Eqn. 2) at which a particle is immobilized in a liquid flowing at velocity (V) in an centrifugal field (the parameters of Eqn. 2 are those defined above where (.rho..sub.p -.rho..sub.m) has been replaced by (.rho.'). These authors further conclude that the contribution of the coriolis force to the net motion of the particle is negligible since it is limited to a tangential plane.
This theory, when applied to Centrifugal Elutriation, (as it was by Sanderson and Bird and by developers at Beckman Instruments) can be utilized in the short term for the separation of cells of different size and/or density. Unfortunately, it is completely inapplicable to long term immobilization of cells or biocatalysts (as is implicit in U.S. Pat. No. 4,939,087) since the theoretical basis is incorrect. As is shown in FIG. 3, there is an additional force acting on the suspended particle which must be taken into account, particularly when the particle is to be immobilized over long time periods (as would be the case in fermentations). That force is a result of the particle's mass. Whereas micro-organisms or animal cells are quite light in weight, their mass is non-zero and consequently gravity will have an effect which is significant, the more so over longer time periods. This is shown more graphically in FIG. 4, where it is shown that there is not a simple description of a radial distance where a particle in an applied centrifugal field can be immobilized in a flowing liquid since the derivation has neglected to consider the effect of gravity on the mass of the particle. The result of this "deviation from theory" is evident in centrifugal elutriation experiments which require prolonged separation times and is shown graphically in FIG. 5. Over longer time periods, the weight of the suspended particles (shown as dark circles in a circular cross-section of a bioreactor chamber in FIG. 5) will cause these particles to settle to the lowest regions of the bioreactor chamber, disrupting the balance of forces which initially suspended them in the chamber. Further, the "aggregation" of these particles into a larger "particle" with virtually the same density as the individual particles results in an increased centrifugal effect which causes the aggregates to migrate to longer radii, eventually clogging the liquid input port.
There are several additional disadvantages to the "Continuous Centrifugal Bioprocessing" art taught by U.S. Pat. No. 4,939,087. First of all, the method is seriously limited by its design to low speed operation (the clockwork-like gear assemblies and moving flexible tube inputs and output lines). This means that the method could not be used either for the culture of low-mass micro-organisms or even large scale cultures of high mass cells, where the required liquid flow rates for adequate nutrition of the cultures would require rotational rates greatly in excess of those allowable by the apparatus in order to provide a counter-acting "centrifugal" force. Next, the method by which gaseous air/carbon dioxide is introduced into the bioreactor chamber (a gas permeable flexible tube in contact with similar flexible tubes which transport input and output liquid flows) will greatly limit the scale of the apparatus since, very rapidly, the required aeration to support cell viability will be limited by the physical pressure and diffusion limits of the flexible tubing. Finally, the apparatus of Van Wie, et al. makes no provision for the vigorous outgassing of, for example, carbon dioxide which will occur as a result of cell metabolism. The metabolically produced gases will: (1) greatly disrupt the input gas exchange necessary for viability by limiting the liquid surface area in contact with the gas permeable tubing; (2) greatly limit the efficient function of the pumping mechanisms necessary for liquid flow into and out of the apparatus; and, (3) result in the growth of gas pockets in the upper portions of the horizontally rotating bioreactor chamber with a resultant decrease of effective bioreactor volume and cell loss by bubble entrainment, as well as serious rotor balance problems.
The prior art demonstrates that while cell immobilization is a greatly desired method for increasing the productivity of living cells in culture, there are a number of drawbacks associated with each class of method. A central problem of all such culture methods is, as Wrasidlo et al. (U.S. Pat. No. 4,937,196) assert, "adequate oxygenation of the cultured cells and removal of carbon dioxide has been a limiting factor in the development of more efficient and economical designs" (see Col. 1, lines 63-65, of U.S. Pat. No. 4,937,196).
Living cells or bio-catalytic subcellular components are unable to derive any benefit from gaseous oxygen. Living cells or biocatalysts derive benefit solely from oxygen dissolved within the aqueous media which surrounds the particles. In batch fermentations which are common for microbial production, the sparging of air or oxygen-enriched gases through the aqueous nutrient media is intended to replace the dissolved oxygen consumed by the metabolizing cells. In this method, most of the gas exits unused while dissolved oxygen levels are maintained at some value. Similarly, the sparging of air (or oxygen) into the nutrient media prior to its use in animal cell culture is intended to maintain a level of dissolved oxygen in the media. While the normal concentration of oxygen in water varies from about 0.2 to 0.3 mM (depending on such factors as pH and ionic strength), it is possible to increase this concentration to as much as 0.5 mM by applying approximately two atmospheres of oxygen pressure over a water solution.
To maintain adequate oxygen concentrations in fermentation media, most of the prior art has focused on increasing the contact between gas and liquid by: (1) producing a very small bubble size (a function of the sparging frit pore size); (2) using high speed agitation to increase the rate of oxygen entrance into the liquid phase; or (3) using a gaseous overpressure of one or two atmospheres above the culture medium to increase dissolved oxygen levels. In the case of animal cell culture, the typical design of animal cell culture chambers has heretofore made it difficult to consider using overpressures greater than a fraction of an atmosphere. Thus, the most common method for increasing oxygen levels employs gas permeable membranes or fibers in contact with flowing nutrient liquid to maintain dissolved oxygen levels. Such methods are taught, for example, by U.S. Pat. Nos. 3,968,035; 4,001,090; 4,169,010; 4,774,187; 4,837,390; 4,833,089; and 4,897,359.
There are a number of problems associated with these methods of increasing the concentration of dissolved oxygen in nutrient media. First and foremost, nearly all of these methods are unable to increase dissolved oxygen concentrations above that obtainable at atmospheric pressure due to the generally fragile nature of other components of the cell culture process. Next, methods which involve vigorous agitation of the liquid-gas mixture to effect increased rates of oxygen dissolution are not applicable to animal cells, which are quite fragile and can easily be damaged by hydraulic shear forces. Finally, those methods which do apply an increased gaseous overpressure above the culture media to increase dissolved oxygen concentrations cannot be scaled up much more than to approximately 1-2 atmospheres of overpressure before it becomes impossible to access the cell-containing liquid media for cell harvest or product isolation without destroying the cultured cells. Nevertheless, the teachings of each of the above methods warrant individual discussion.
U.S. Pat. No. 4,897,359 (issued to Oakley, et al.) discloses a method for oxygenating animal cell culture media for subsequent introduction into cell culture vessels in which an oxygenated gas, at an indeterminate pressure, is passed through a multiplicity of gas permeable tubes surrounded by the liquid medium to be oxygenated. While the pressure of the input gas may be above atmospheric pressure, the pressure of the oxygenated exit liquid can be no more than atmospheric pressure. If the oxygenated exit liquid were above atmospheric pressure, it would result in outgassing of the liquid medium when the medium was introduced into the typical cell culture vessel. Such outgassing would also result in bubble formation within the media, which would be extremely deleterious to animal cell viability. Thus, the method of the invention of Oakley, et al. is useful only in assuring that the cell culture media possesses the maximum dissolved oxygen concentration obtainable at atmospheric pressure.
U.S. Pat. No. 4,837,390 (issued to Reneau) discloses a method of preservation of living organs (for subsequent transplant) in which hyperbaric conditions (2 to 15 bars or 29 to 218 pounds per square inch (psi)) are maintained. In the Reneau method, a living organ is placed in a chamber capable of withstanding pressure and a perfusion liquid containing nutrients is pumped into and out of the chamber while a gaseous oxygen overpressure is also applied to the chamber. The method does not discuss cell culture or fermentation.
U.S. Pat. No. 4,833,089 (issued to Kojima, et al.) discloses a cell culture method in which a gaseous overpressure of oxygen or air is applied over a stirred liquid media in which cells are cultured. In this method, the pressure limitations of the apparatus (which include peristaltic pumps, flexible low-pressure pump tubing, and low pressure filter apparati) necessarily limit the method to overpressures of 0.3-0.7 kg/cm.sup.2 (approximately 4.3-10 psi). Thus, the concentration of dissolved oxygen in the media bathing the cells is limited to values only slightly greater than that obtainable at atmospheric pressure (Col. 4, lines 15-17).
U.S. Pat. No. 4,774,187 (issued to Lehmann) discloses a method for the culture of microbial cells in which a gaseous overpressure is applied over stirred liquid media in which cells are cultured. In this method, the gaseous overpressure makes it impossible to access the interior of the culture compartment without depressurization and cell destruction. The inventor overcomes this problem by raising an overflow line from the media-containing bioreactor to a height such that the liquid pressure of this overflow line equals the gas overpressure. By the establishment of a siphon originating in the elevated overflow vessel connected to the overflow line, one may withdraw liquid or cells from the culture chamber without depressurizing the chamber. Because the typical culture medium is essentially an aqueous solution, the system pressure is limited to the height of a column of water which would balance the system pressure. Thus, for example, at a system pressure of 37 psi (gauge), a column of water approximately 50 feet in height would be required. Thus, the method from a practical standpoint, is limited to dissolved oxygen levels obtainable at 1-2 atmospheres of overpressure.
U.S. Pat. No. 4,169,010 (issued to Marwil) discloses a method for improved oxygen utilization during the fermentation of single cell protein in which a gaseous overpressure above a stirred nutrient liquid in a bioreactor containing the growing cells is utilized to increase oxygen delivery to the growing cells. In this method, the recirculation of cell-free media (lean ferment) obtained by centrifugation of the bioreactor contents is passed back into the bioreactor through an absorber section containing a gas contacting zone. The gaseous overpressure is maintained by a gas pressure regulator device which blocks pressure release or vents the gas in response to a desired dissolved oxygen sensor setting. The patent discloses overpressures of about 0.1 to 100 atmospheres (approximately 16.2 to 1485 psi) (Col. 7, lines 28-30, of U.S. Pat. No. 4,169,010). The inventor states that a maximum desirable gaseous overpressure of 1 to 2 atmospheres is preferable.
Presumably, the reason why a maximum desirable gaseous overpressure of 1 to 2 atmospheres is preferable in the Marwil method, and would be difficult to exceed, arises from the fact that the metabolizing cells also release carbon dioxide, a metabolite which must be removed from the nutrient media by gas evolution if cell viability is to be maintained. Gas overpressures greater than 1 to 2 atmospheres utilized to increase dissolved oxygen content would necessarily result in very large dissolved carbon dioxide levels retained within the nutrient media which could not be removed until the gaseous overpressure was released. It should be noted that carbon dioxide solubility in aqueous solution is approximately an order of magnitude greater than that of oxygen. The inability to remove dissolved carbon dioxide from the media while still delivering increased oxygen to the media would cause a decrease in aqueous pH, a serious problem of the method of this patent. In addition, the method of Marwil is designed solely for the continuous harvest of cells; the method cannot be applied to the continuous harvest of the aqueous solution which might contain an excreted cellular product chemical.
U.S. Pat. No. 4,001,090 (issued to Kalina) discloses a method for microbial cell culture which incorporates a process for improved oxygen utilization very similar to that outlined above for Marwil (U.S. Pat. No. 4,169,010). The method of Kalina directly addresses the problem of carbon dioxide removal mentioned earlier in connection with the method of Marwil. This problem is eliminated by the inclusion of a gas-liquid separator in the fermentor circuit. In the method of Kalina, an oxygenated gas at an unspecified pressure greater than atmospheric is released into the fermentation chamber at its bottom (common sparging). However, by means of a backpressure device, the media is maintained at an overpressure of as much as 3 to 3.5 atmospheres (44.1 to 51.5 psi) to provide both a motive force for the media recirculation as well as to aid in the removal of excess gas distal to the fermentation zone (Col. 4, lines 35-37). The Kalina process relies heavily on the presence of gas bubbles for the agitation of the media and is suitable solely for use in microbial cell fermentation; the method could not be applied to animal cell culture because such cells are extremely sensitive to hydraulic shear forces and are damaged or destroyed by contact with air-water interfaces such as those encountered in gas bubble-containing media.
U.S. Pat. No. 3,968,035 (issued to Howe) discloses a method for the "super-oxygenation" of microbial fermentation media in which the common sparging of an oxygen-containing gas into the fermentation media is replaced by the introduction of this gas into an "oxidator" vessel in which high-shear agitation is used to reduce the average size of the gas bubbles, thus increasing the available surface area for gas-liquid contact with the result that maximal dissolved oxygen concentration is maintained. The fermentation media which has thus been treated is pumped into the fermentation reactor while exhausted media from this same source provides the input to the "oxidator" vessel. The process of this invention thus provides a combined liquid and oxygen-enriched gaseous mixture to the culture chamber, a situation which is inapplicable to animal cell culture for the previously-mentioned reasons.
Finally, because the immobilization of cells or microorganisms requires that a cell confinement chamber is part of the process system, the recent literature has been examined for comparison. There are a number of cell culture chambers in existence. Many of these chambers provide for the input and output of a liquid stream, several have viewing ports, and all provide a surface upon which cells may attach or a chamber in which suspended cells may be cultured. Such methods are taught, for example, in U.S. Pat. Nos. 3,871,961; 3,753,731; 3,865,695; 3,928,142; 4,195,131; 4,308,351; 4,546,085; 4,667,504; 4,734,372; 4,851,354; and 4,908,319. In all cases, the operating pressure of these confinement chambers is one atmosphere (or less) and thus these chambers are unsuitable for processes in which increased dissolved oxygen levels are desired and are necessarily limited to those dissolved oxygen levels obtainable at atmospheric pressure.
The current state of the art reveals that there are three inter-related problems which plague the economic use of mass cultures of microbes, animal cells, or their subcellular components: (1) As is evident from the sheer volume of the prior art on cell immobilization, the focus of much work has been to increase the density of the cell culture. It is obvious that the economical production of a biological product will be directly related to the ability to efficiently culture large aggregates of the desired cell type. Unfortunately, the drive to increase cell culture density leads to the evolution of the two secondary problems: (2) the inability to adequately nutrition a high density cell aggregate; and (3) the inability to supply adequate oxygen to high density aerobic cell populations. As cell density is increased, the only method for the supply of adequate liquid nutrient to the aggregate involves increased liquid flow rates which, in all cases in the prior art, eventually limits the overall scale of the immobilization method. Similarly, as the cell density increases, the inability to deliver adequate dissolved oxygen (or any other gas) to the cell aggregate is even more of a limiting factor and severely reduces the scale of the culture.
Accordingly, there remains a need for an apparatus and method for continuously culturing, feeding, and extracting biochemical products from either microbial or eukaryotic cells or their subcellular components while maintaining viable, high density aggregates of these biocatalysts. In addition, there is a need for a method for the absolute immobilization of sample biocatalyst populations which will allow the study of various nutritive, growth, and productive parameters to provide a more accurate understanding of the inter-relationships between these parameters and their effects on cell viability and productivity.